1. Field of the Invention
The present invention relates to a method for producing particles using compressive fluid, a method for producing a toner, and an apparatus for producing particles.
2. Description of the Related Art
Conventionally, various products in the shape of particles are produced by processing resins, such as thermoplastic resins and thermosetting resins, depending on the properties thereof. For example, a toner used in an image forming apparatus is produced by heating a raw material containing a thermoplastic resin to a temperature equal to or higher than the glass transition temperature Tg thereof for plasticization, or to a temperature equal to or higher than a melting point thereof for melting, followed by forming the plasticized or melted raw material into particles.
Specifically, a kneaded product, which is produced by melting and kneading the raw material containing the thermoplastic resin, is cooled to be a solid, and then pulverized, to produce a toner in the form of particles (Japanese Patent (JP-B) No. 2677685). However, when the toner is produced by pulverization, it is difficult to control particle size. When fine powder is not sufficiently removed, the basic properties of the toner, such as charging ability, fixing ability, and heat resistant storage stability (change over time) are degraded.
It has been known that in order to control a toner particle shape, a material containing a thermoplastic resin is kneaded and pulverized, and then dispersed in an aqueous solvent in the presence of hydrophilic inorganic fine particles, followed by heating the mixture to remove the solvent, to thereby produce a toner in the shape of particles (Japanese Patent Application Laid-Open (JP-A) No. 09-34167). However, in this method, since the solvent is used, it is necessary to dry the solvent, and waste liquid is formed. Since in the kneading and pulverizing step, the thermoplastic resin is processed at high temperature and with high shear, problems occur, such as color change, degradation of the above-described basic properties due to degradation of a resin, and the like.
Recently, a toner produced by Particles from Gas Saturated Solutions (PGSS) process has been proposed. The PGSS process is performed in such a manner that a thermoplastic resin is melted, and the compressive fluid is brought into contact with the melted thermoplastic resin, and then mixed using a static mixer, followed by decompressing and expanding the resultant mixture, to thereby produce a toner (JP-B No. 4113452). According to this process, since the compressive fluid is used, a toner can be produced without using a solvent, as well as with no need to pulverize the thermoplastic resin.
However, by the proposed PGSS process, the size of the resultant particles becomes large, i.e., several hundred micrometers to several millimeters. Moreover, particle size distribution becomes broad and nonuniform because of fibrous products and cohesion of toner particles. Thus, it is difficult to produce a large amount of uniform fine particles having small particle sizes. For example, in an application, which needs particles having narrow particle size distribution, such as an electrophotographic toner, classification is necessary, and yield is degraded. When a temperature of a mixture is increased, and the molecular weight of the thermoplastic resin as a raw material is decreased, in order to improve processability, there occur problems that the thermoplastic resin is degraded, causing degradation in the basic properties of the toner, such as charging stability.